Certain high voltage power supply applications require that a capacitor be charged to a high voltage, and subsequently rapidly discharged through a device. Examples include xenon flash-tubes (such as are commonly used in lab strobes, photo-flash units, etc.), pulsed lasers (medical, industrial, military, and scientific applications), and other types of discharge devices. Discharges from solid state pulsed lasers, for example are often accomplished by firing a flash tube surrounding a solid state laser rod.
Capacitors are often used for such applications because high voltage capacitors are relatively easy to obtain, and because capacitors can be discharged extremely rapidly (by drawing current from the capacitor). (Other applications which discharge the capacitor more slowly are also possible.) The frequency with which the capacitor may be discharged, i.e., the discharge rate (e.g., pulse rate on a strobe light or laser, frame rate on a camera photo-flash unit) is limited, at least in part, by the rate at which the capacitor can be charged (made ready for discharge) by an associated charging power supply.
As is well known, the voltage across a capacitor increases as the product of the capacitance value of the capacitor and the integral with respect to time of the current through the capacitor. Consequently, the greater the capacitance, the longer it takes to charge the capacitor through a given voltage increase across the capacitor for a given charging current. Conversely, the greater the charging current, the more rapidly a capacitor can be charged. The charge (in coulombs) on a capacitor is equal to the product of the voltage across the capacitor and the value of the capacitor. The amount of electrical energy stored in a capacitor (in joules or watt-seconds) is equal to one half the product of the capacitance value and the square of the voltage across the capacitor.
High output devices, e.g., high output pulsed lasers, require corresponding large amounts of energy from each discharge of the capacitor from which they receive their pulse energy. Although it is certainly possible to build up the energy in the capacitor (charge the capacitor) slowly, this is often undesirable. Many applications (e.g., medical surgical lasers) require rapid pulsed operation. Rapid pulsed operation requires rapid charging of the capacitor, requiring correspondingly high current from the capacitor-charging power supply.
If a conventional voltage regulated power supply were to be used to charge energy storage capacitors, an inordinately large and heavy power supply would be required. In order to prevent excessive current surges (high inrush currents which occur upon connection to a completely discharged capacitor), an inductive choke must be connected in series with the charging current. The choke is typically large and heavy in its own right. As a result of the high weight of the power supply, such systems are not well suited for applications such as airborne pulsed laser systems. Resistively charged systems exhibit relatively low efficiency, which can be a significant disadvantage in a battery powered system (choke systems can be efficient, but resistive systems have 50% typical efficiency.
A capacitor charging system which addresses the problems of surge current and size, in part, is described in U.S. Pat. No. 3,654,537, (hereinafter "COFFEY"), incorporated by reference herein. COFFEY describes a technique whereby an energy storage capacitor is charged in a series of successive steps, each of a higher voltage level than the preceding step. According to COFFEY, power from each of a series of unequal secondary windings of a transformer is rectified and selectively switched into a stacked series arrangement. By selectively enabling (switching in) the windings in various combinations, the capacitor can be charged in a number of controlled voltage steps effectively limiting surge currents without requiring large inductors. Current into the capacitor is maintained at a relatively constant level over the charging cycle. Further, charging efficiency is considerably improved.
A disadvantage of COFFEY is that it does little to increase the output current of the supply into the capacitor. Another disadvantage of COFFEY, is that at any given time, a number of the windings of the transformer may not be in use, implying a rather poor size to charging-rate ratio for the transformer. Since the rate of energy transfer into a capacitor for a given current is proportional to the voltage across the capacitor, the COFFEY technique transfers the most energy in the last part of the charging cycle (which makes intuitive sense, since the last part of the cycle is when more of the secondary winding are switched in). As a result, the load seen at the primary of the transformer increases throughout the charging cycle.
Another approach is shown in Japanese Patent No. 1-286800 (hereinafter TAKENAKA). In TAKENAKA, a pair of windings of a generator block are first connected in parallel, and then in series. The output is rectified and used to charge a capacitor. When the capacitor is in its fully discharged state, the parallel winding combination is used, providing roughly double the current which could be supplied by a single winding alone. When the voltage across the capacitor reaches a critical point (related to the single winding voltage for the transformer) the two windings are switched into a series configuration, halving the current carrying capacity of the combined windings, but doubling the voltage. The rectified voltage from the series combination charges the capacitor for the remainder of the charging cycle.
The TAKENAKA approach has the advantages (e.g., over COFFEY) of maintaining both windings "active" at all times, and providing a more constant and optimal rate of energy transfer to the capacitor, thereby improving the size to charging-rate ratio for the transformer and speeding up the charging cycle. Nevertheless, the TAKENAKA approach suffers from significant disadvantages. In order to switch the (two) windings from a series to a parallel configuration, three FET (field effect transistor switches) are required. The controls for these switches can be rather cumbersome and expensive. Further, since the rectifier must handle both the series and parallel coil winding combinations, it must be capable of handling both high current and high voltage.